Typically, the performance of an electronic system is limited by the noise emitted by the system's components. For example, the performance of a wireless telecommunications terminal is typically limited by the quantity of phase noise produced by its oscillator.
Although a crystal oscillator typically emits little phase noise, it cannot be easily fabricated as a component of an integrated circuit. In contrast, an oscillator that is fabricated from semiconductor devices in an integrated circuit typically emits more phase noise than does a crystal oscillator.
Although there are several types of noise emitted by a semiconductor device (e.g., thermal noise, shot noise, flicker noise, etc.), empirically, the largest contributing factor to phase noise in a circuit is flicker noise, which is also known as "1/f" noise because the output current noise spectral density of the device varies inversely with frequency.
Empirically, the quantity of flicker noise emitted by a device is affected by the process that is used to fabricate the device. Therefore, a semiconductor manufacturer can affect the quantity of flicker noise emitted by the devices it fabricates by carefully measuring the flicker noise emitted by the devices emanating from a fabrication line and thereafter modifying the fabrication line process in response to the measured results.
Typically, the magnitude of the noise emitted by a device is too small to be directly observed by a noise measurement apparatus, such as a spectrum analyzer, and, therefore, the noise is amplified by an amplifier prior to analysis by the noise measurement apparatus. In most cases, however, the magnitude of the noise emitted by the device under test is small in comparison to the noise emitted by a typical amplifier, and, therefore, it is paramount that the amplification is performed in such a manner that the noise emitted by the device is not engulfed by the noise emitted by the amplifier. Therefore, the amplifier must itself emit very little noise.
FIG. 1 depicts a schematic diagram of a two-stage, low-noise amplifier that is well-known in the prior art. The first stage comprises bipolar junction transistor 101 and the second stage comprises field-effect transistor 102, interconnected as shown. As is well-known in the prior art, a two-stage amplifier comprising a bipolar junction transistor and a field-effect transistor has the same gain regardless of which transistor is used in either stage. For example, the overall gain of a two-stage amplifier, G, is related to the gain of the first stage, G.sub.1, and the gain of the second stage, G.sub.2, by the function: EQU G=G.sub.1 +G.sub.2 (Eq. 1)
Yet Friis' formula, which relates the noise emitted by a two-stage amplifier to the noise emitted by each stage, suggests that the first stage should comprise the bipolar junction transistor and the second stage should comprise the field-effect transistor. For example, the noise factor of a two-stage amplifier, F, is related to the noise factor of the first stage, F.sub.1, and the noise factor of the second stage, F.sub.2, by Friis' formula: ##EQU1## which clearly indicates that the noisier device should be used in the second stage. Because it is well-known in the prior art that field-effect transistors are noisier devices that bipolar junction transistors, Friis' formula suggests that the first stage should comprise the bipolar junction transistor and the second stage should comprise the field-effect transistor.
The amplification of extremely small signals, such as flicker noise, requires an amplifier with high gain, a large bandwidth, low noise, high input impedance, high common mode rejection and immunity to external noise sources. So although the amplifier depicted in FIG. 1 is characterized by high gain and a large bandwidth, it does not have a high input impedance. The high input impedance can be achieved with a field-effect transistor in the first stage and the high gain can be achieved with a bipolar junction transistor in the second stage, but Friis' formula, as discussed above, suggests that such an arrangement will unnecessarily introduce noise into the signal.
Therefore, the need exists for an amplifier that has low noise, high gain, a large bandwidth, high input impedance, high common mode rejection and immunity to external noise sources.